Glycation is the non-enzymatic addition or insertion of sugar molecules into proteins, DNA, and lipids that occurs in a biological environment. This reaction causes damage to proteins or other molecules and is considered to be a significant contributor to many diseases of aging (1).

Introduction

Since the early 1900s, scientists have known that heating proteins in the presence of sugars results in the formation of new cross-linked chemical bonds (2). This reaction is the process by which toast and many baked or cooked foods turn brown. Maillard, and later Amadori, described this formation of complexes between sugars and the amino acids of proteins that were believed to cause the toughening and discoloration of food during the cooking process and after prolonged storage (3). Proteins cross-linked with sugars are considered to be causative factors in many age-related and diabetic disorders.

Glycation  The Process and Effects

It was long believed that glucose, the type of sugar most abundantly contained in an organism, was biologically inert. Glucose is capable of reacting with proteins, which can also react with DNA and lipids. These chemical reactions called glycation do not involve enzymatic processes.

The term "glycation" is the non-enzymatic glycosylation of proteins, nucleotides and lipids by saccharide derivatives. It is thought to contribute to the development of chronic vascular complications of diabetes, non-diabetic nephropathy, vascular disease, Alzheimers disease, and aging.

The glycation of proteins leads to covalent cross-linking and abnormal structural stabilization of extracellular matrix proteins. These harmful consequences of this cross-link formation in humans was proposed by Brownlee as an outgrowth of a research effort focused on diabetes (4). Cross-links are now considered to be likely causative factors in the development of many age-related and diabetic disorders, particularly those associated with the cardiovascular and renal systems, by causing biochemical and structural alterations on proteins.

Recent studies have led to the hypothesis that formation of these amino-sugar structures was a step along a new biochemical pathway in which permanent glucose structures, called "advanced glycosylation end products" (AGEs), were formed on the surface of proteins (2). These AGEs were able to further interact with adjacent proteins to form permanent links between proteins. The AGE cross-link has been found to be unique in biology and is prevalent in animal models of diabetes and aging. Figure 1 shows the AGE Pathway leading to the formation of pathological AGEs.

Amadori

rearrangement

+RNH2

Glucose Schiffs Base Fructosamine

-RNH2

Figure 1. The AGE Pathway

Glycation is an unavoidable process of metabolism. In certain physiological states, one or more of the following changes to glycation-related processes occurs: the rate of glycation is increased, the renal clearance of AGEs is decreased and/or the expression of AGE receptors is increased. This may lead to AGE-related basement membrane thickening, AGE-mediated cell activation and amyloidosis. Fructosamines are indicators of medium-term glycamic control and AGEs are indicators of medium to long-term glycamic control in diabetes mellitus. Some AGEs are risk markers and others are risk factors of disease (4).

The AGE Pathway represents one of several pathological processes believed to be responsible for aging.

Figure 2 shows the chemistry involved in the conversion of glucose and an amino acid to an AGE.

Figure 2. The Chemistry of AGE Formation (from Reference 5)

Glucose, in its aldehyde form, reacts with the amino groups of protein to form a Schiff base which rearranges to a stable ketoamine adduct (6). This process, non-enzymatic glycation of protein, occurs naturally in the body. Glycation not only affects the structure and function of protein, but also initiates a series of Maillard or browning reactions that eventually lead to cross-linking and denaturation of proteins. All proteins in the body are subject to these reactions, but long-lived proteins such as collagen, which represents over 30 percent of body protein, accumulate chemical damage with age. The increased rate of glycation of collagen during hyperglycemia is implicated in the development of complications of diabetes, such as blindness, renal and vascular disease.

Glycation of proteins is implicated in diabetes (7). Glucose and other saccharides are important glycating agents, but the most reactive glycating agents are the a
-oxoaldehydes, glyoxal, methylglyoxal and 3-deoxyglucosone. Cross-linking potency is variable among the sugars, with a rank order of glucose < fructose < ribose, and phosphorylated sugars being more potent than their unphosphorylated counterparts (8). Early glycation adducts (Schiffs base, fructosamine) and advanced glycation adducts (AGEs) are formed (Figure 1). Cross-linked proteins appear to be involved in both diabetes and senescence.

This type of reaction is somewhat slower in an organism. The sugar-protein complex is able to initiate a chain reaction that ends in forming reversible intermediate substances in a few days. These substances dehydrate, condense and reorganize themselves after a few weeks and become the irreversible components of AGEs.

The AGEs induce irreversible cross-links between molecules and thus alter their chemical and biological properties. Most of the carbohydrate-derived products which accumulate in tissue proteins with age and accumulate at an accelerated rate in diabetes are products of both glycation and oxidation reactions. Some oxidation reactions appear as glucose-induced damage to tissue proteins. Age-corrected levels of glycoxidation products in collagen correlate with the severity of diabetic complications (9).

Also, cross-linking of collagen proteins, for example, contributes both to the rigidity and the loss of elasticity of tissues, and to the thickening of capillary walls observed in diabetes and during the aging process. This protein modification is also responsible for crystalline lenses becoming opaque in cataracts, a degenerative disease that is also frequent in diabetic or aged persons. The glycation of nuclear acids may be the cause of DNA mutations and could alter its capacity for replication and transcription. The combination of AGEs and lipids increases oxidation rates and hastens the development of vascular lesions.

In addition to the cross-linking of long-lived molecules, AGE are able to stick the rapidly renewable plasma molecules together, whether albumin, antibodies, or LDL cholesterol.

The cross-linking of proteins and the trapping of various molecules by the AGEs contribute to developing atherosclerosis, kidney, vascular and neurological diseases in both diabetes and the aging process. Also glycation substances may be involved in the pathogenesis of Alzheimer's disease, since an accumulation of these substances is observed at the sites of neuronal degeneration during the course of this disease (10,11).

The body does have a defense against cross-linked proteins. The immune system has macrophages with special receptors for AGEs. The macrophages engulf AGEs and eventually the products are excreted in the urine (12).

Future research could involve the identification of critical AGEs and AGE receptors involved in protein cross-linking and cell activation. It seems important to characterize their participation in disease mechanisms and to develop therapeutic agents to counter glycation-mediated pathogenesis. Interestingly, carnosine, (beta-alanyl-L-histidine) a naturally occurring molecule is an aldehyde scavenger and appears to be protective against some cross-linking (13). Also, ethanol consumed in small quantities appears to protect mice hemaglobin against AGE products (14). Thecompany (Alteon) is developing pharmaceutical compounds that interfere with the cross-linking process or even break existing cross-linked proteins. One such "AGE breaker", ALT-711 is awaiting FDA approval for human use (15).

Summary

In 1912, Louis Camille Maillard, a food chemist, discovered this chemical reaction of glucose and other sugars with proteins. Since then, chemists have known about the glycation process, but only recently have biologists accepted the fact that these so-called Maillard reactions could occur within an organism.

Maillard reactions are commonly observed as they are responsible for the browning of meat when cooking. Heat considerably accelerates the speed of the reaction between the glucose and the proteins contained in meat or bread, when toasted.

These resulting cross-linked proteins accumulate in diabetic patients and aged persons. AGEs are also implicated in many pathological conditions commonly found in these groups of people.